1. Field of the Invention
The present invention relates to optics. More specifically, the present invention relates to systems and methods for directing and correcting high-power beams of electromagnetic energy.
2. Description of the Related Art
Directed energy weapons and specifically high-energy laser (HEL) weapons are being considered for variety of military applications with respect to a variety of platforms, e.g., spaceborne, airborne and land based systems to name a few. These weapons generally involve the use of the laser or other source of a high-energy beam to track and destroy a target. To achieve mission objectives, directed energy weapons must be accurately steered and optimally focused. Steering involves line-of-sight control while focusing, with respect to HEL weapons, involves wavefront error correction. Currently, wavefront error correction is typically achieved using adaptive optics. The current state of the art in laser beam control adaptive optics requires placing one or more deformable mirrors within the highest intensity portion of the beam path. The conventional deformable mirror is typically a large element with a thin face sheet and a number of piezoelectric actuators. Actuators are located behind the face sheet and push and pull on the surface thereof to effect the deformation required to correct wavefront errors in an ongoing beam. The size of the active region of the deformable mirror must accommodate the full size of the high power laser beam in the high power Coudé path prior to expansion via an output telescope.
In addition, one or more fast (high temporal bandwidth) steering mirrors may be used to correct for tilt and direct the line-of-sight. A lower-bandwidth course gimbal may also be employed to correct for line-of-sight errors as well. A plurality of wavefront sensors are typically employed along with an aperture sharing element (ASE). The ASE allows a single optical path and aperture to be advantageously used for both the low power sensors and the high power output beam, ensuring that the corrected path is the same as that taken by the high power beam.
Unfortunately, the use of delicate optical devices in the path of a high-power energy beam is problematic. This is due to the fact that the high-energy the beam will heat and distort the optical element unless the element is actively cooled or has a coating with a very low optical absorption coefficient. The most durable coatings require a high temperature application process. Deformable mirrors are typically coated after the face sheet is bonded to the actuators, which limits the maximum temperature to which the deformable mirror assembly may be exposed without degrading the bond. Therefore, coatings may need to be applied at lower than optimal temperature using more complex coating processes, thereby reducing durability and/or increasing manufacturing cost.
In addition, conventional adaptive optics systems using deformable mirrors are limited in performance. Conventional deformable mirrors systems are limited with respect to the speed at which the mirror drive signals are computed and the reaction speed of the deformable mirror mechanism to correct for aberrations. There is also a limitation with respect to the number actuators that can be used. The number of actuators that may be used determines the resolution or “order” of the mirror. The stroke of the conventional deformable mirror is limited. “Stroke” relates to the amount of mirror surface deflection that may be achieved before either the piezoelectric actuators exceed their dynamic range or the face sheet begins to fail. Further, a conventional continuous face sheet deformable mirror cannot correct for a pathology in the spatial phase pattern, such as a branch point or an abrupt phase discontinuity. A branch point is a “singularity” in a deeply scintillated phase pattern caused by atmospheric turbulence over a long propagation path in which the phase monotonically increases around a zero amplitude point like a corkscrew, thereby requiring an abrupt 2π phase correction within the spatial phase pattern. Abrupt phase discontinuities may be caused by the optical discontinuities between segments of a multi-segment primary mirror.
In U.S. Pat. No. 5,694,408, issued Dec. 2, 1997, (the teachings of which are incorporated herein by reference), Bott, Rice, and Zediker appear to disclose a scheme which allows the deformable element to be placed in the low intensity region between a master oscillator and an array of fiber power amplifiers. The approach is to pre-distort the phase of the oscillator beamlets after separation in a distribution network and before injection into the fiber amplifier array, such that the pre-distortion corrects both the piston error between the individual fibers and optical aberrations in the atmosphere. However, this scheme is practical only with a coherently-combined array of single-mode fiber amplifiers, as each fiber channel is correctable in piston only, not high order. Also, this scheme is not applicable to multi-mode laser media such as large core fiber amplifiers or bulk media lasers as contemplated for weapon class HEL devices and may not be scaleable to high power levels due to random, high frequency phase noise caused by pump-induced temperature fluctuations within the fibers.
In U.S. Pat. No. 5,090,795, issued Feb. 25, 1992, the teachings of which are incorporated herein by reference, O'Meara and Valley appear to disclose several related schemes for using a liquid crystal light valve (LCLV) in a self-correcting adaptive optics system. This approach, however, places the LCLV in the high power beam path and is therefore limited by the damage susceptibility of the liquid crystal material.
To be effective and affordable, a space based Laser HEL beam director, for example, may have a lightweight primary mirror that is larger than the shroud diameter of the launch vehicle. This requires a mirror design that is collapsible during launch and deployable upon release in orbit. Such a deployable segmented mirror will have significant figure and static and dynamic piston phase errors due to the low stiffness pedals and physical arrangement of the deployment mechanism.
A method has been developed for sensing the outgoing wavefront error in a primary mirror that uses holographic optical elements (HOEs) fabricated on the primary mirror surface, see for example G. Golnick, “Directed Energy Systems”, The Infrared and Electro-Optical Handbook, Volume 8, Chapter 5, ERIM, Ann Arbor, Mich., pp 441-442 (1993) for a description of primary mirror wavefront sampling using holographic optical elements. This wavefront sensing approach has also been applied to large segmented primary mirrors for space applications. The adaptive optics subsystem designs, to date, utilize the sampled outgoing wavefront from these HOEs, but close a conventional servo-loop around conventional continuous face sheet deformable mirrors which are inserted in the Coudé path of the high power beam. Unfortunately, this approach is limited by the performance of conventional deformable mirror technology, particularly the limited stroke and inability to accommodate discontinuities in phase created by the pedal joints. This approach requires that the segmented optical element maintain absolute phase and limited segment-to-segment tilt to remain within the control capability of the deformable mirror.
In U.S. patent application Ser. No. 09/965,764, filed Sep. 28, 2001 by R. W. Byren and A. F. Trafton and entitled SYSTEM AND METHOD FOR EFFECTING HIGH-POWER BEAM CONTROL WITH ADAPTIVE OPTICS IN LOW POWER BEAM PATH, the teachings of which are incorporated by reference herein, Byren and Trafton describe several beam control architectures which use the wavefront reversal property of nonlinear phase conjugation to place a photonic deformable element in a low-power master oscillator beam path to perform the adaptive optic correction primarily for tactical HEL applications. Unfortunately, while effective when integrated local- and target-loop adaptive optics are used, this architecture does not adequately address the needs of current and proposed space based applications.
Accordingly, a need remains in the art for a system and method for effecting outgoing wavefront sampling and correction for space based and other HEL applications.